1. Field of the Invention
This invention relates to a device for generating ultraviolet radiation of high radiation intensity, in which the radiation can be generated thermoemissively, in a discharge tube which transmits UV radiation and is provided with a filling of mercury/rare gas and has an internal diameter of between 4 and 20 mm, by means of a gas discharge which is wall-stabilised and takes place between two electrodes under a pressure of mercury between 5.times.10.sup.-3 and 5.times.10.sup.-1 Torr and a pressure of rare gas between 0.01 and 10 Torr and at a current density of the discharge current of the gas discharge between 1 and 25 A/cm.sup.2, the two ends of the discharge tube being joined to tubular envelopes for receiving the electrodes.
2. Description of the Prior Art
Devices of this type, called high-current low-pressure UV radiation sources, have for example been disclosed in German Offenlegungsschriffen Nos. 2,412,997 and 2,433,557. To achieve a high radiation intensity, lamps of this type are usually operated with direct current. This requires two different electrodes, namely a thermoemissive cathode, e.g. a barium oxide cathode, and an anode which receives electrons of high energy and is therefore extremely resistant to high temperatures, e.g. an anode consisting of graphite. Furthermore, since a large stream of mercury vapour must flow from the anode space to the cathode space, when the radiation source is operated at a rather small pressure difference, it is necessaery to provide a pressure-equalizing space. However, electrophoretic processes in the discharge cannot be avoided so that the radiation emission is not uniform over the entire surface of the discharge space and premature ageing of the radiation source occurs.
German Offenlegungsschrift No. 2,515,607 has disclosed a high-current mercury low-pressure lamp which has a pressure-equalizing space and in which the anode electrode and the cathode electrode are located one behind the other in an envelope, one of the two arms of the discharge space coaxially protruding into the envelope and the anode of annular shape passing through. Lamps of this type are preferably used as sources of ultraviolet radiation, for example for the sterilization of packaging materials. When these lamps are used in practice, it has been found that a direct arc-through between the cathode and anode can occur through the pressure-equalizing space and that as a result the entire lamp is destroyed under certain circumstances. The reason for this arc-through is the excessive temperature of the pressure-equalizing space, the shape of which in the known lamps is determined essentially by two parts in the shape of collars. The high temperature in the pressure-equalizing space in these lamps results from the fact that this space is heated by the radiation from the cathode. In addition, heating is effected by the anode column of the discharge.